1. Field of the Invention
The present invention relates generally to a Digital Pre-Distorter (DPD) of a power amplifier, and more particularly, to a method and an apparatus for improving stability and performance of a DPD by adding a Saturation Level Limiter (SLL) to the DPD and preventing saturation of an output peak signal of a power amplifier.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a conventional digital pre-distortion system utilizing a digital linearization.
Referring to FIG. 1, a Digital Pre-Distorter (DPD) 110 of a digital unit 100 compares an input signal with an output signal in order to linearize an output of a power amplifier 120, minimizes a distortion component of an output signal, and allows an input modulation signal to be linearly amplified.
To perform extraction of a distortion component of the power amplifier 120 and perform a correction algorithm of a corrector 135 for an operation of the DPD 110, an input signal representing an entire signal and an output signal thereto are required. That is, the corrector 135 receives an input signal and an output signal of a down-converter 130, and provides a coefficient for digital pre-distortion to the DPD 110. Basically, the coefficient sets the DPD 110 gain such that the overall combination response of the power amplifier 120 and the DPD 110 becomes a linear system. This means that the DPD 110 is actually acting as an inverse power amplifier non-linearity pre-equalizer.
An up-converter 115 up-converts an output signal of the DPD 110 to provide the up-converted signal to the power amplifier 120, and the down-converter 130 down-converts an output signal of an attenuator 125, which attenuates an output signal of the power amplifier 120 by a predetermined level, to provide the down-converted signal to the corrector 135.
FIG. 2 illustrates a conventional power amplifier and a conventional power amplifier in a digital pre-distortion transmission system. Specifically, FIG. 2 illustrates a conventional power amplifier 150 with no digital pre-distortion operation applied to the input thereof and a power amplifier 120 with digital a digital pre-distortion operation applied to the input thereof by DPD 110.
When the gains of power amplifiers 150 and 120 are normalized to 1, input/output characteristics for ports 1, 2, 3, and 4 are illustrated in FIG. 3.
FIG. 3 is a graph illustrating input/output characteristics of the conventional digital amplifiers illustrated in FIG. 2.
Referring to FIG. 3, a solid line A represents the input/output characteristics ([1]:[2] before linearization, [3]:[4] after linearization) of the power amplifiers 150 and 120.
When an inverse function of these input/output characteristics is determined, a curve like a dotted line C is obtained. This curve represents input/output characteristics ([1]:[3]) of the DPD. When an input signal [1] is applied, an output signal of the DPD becomes [3]. An output signal of the power amplifier to which the signal [3] has been input becomes a signal [4] that has been linearized for the signal [1].
A solid line B represents an input/output characteristic [1]:[4] of a system after linearization. A saturation power level POUT—sat of the power amplifier may be known via a power amplifier input/output signal [3]:[4] after linearization.
The saturation power level POUT—sat of the power amplifier is a value whose gain has dropped down by Gcomp (gain compression) compared to an output value when distortion of the power amplifier does not exist.
Examination of X-axis of a solid line B via the saturation power level POUT—sat of the power amplifier shows a system linearization input threshold PIN—th (input power threshold value) that can be linearized. In a system input value of more than the PIN—th, an output after linearization is saturated like the solid line B.
Upon occurrence of a saturation phenomenon, when linearization iteration time of the DPD is increased, a saturation phenomenon of the power amplifier increases like a dotted line of the end portion of the solid line A.
Due to this, the DPD obtains a more accurate inverse function and so has a steeper slope in a region exceeding PIN—th as in the dotted line C (DPD input/output characteristics) of the drawing.
FIG. 4 is a graph illustrating input/output characteristics of the conventional digital amplifiers illustrated in FIG. 2, when a system input value of PIN—th or more is applied.
Referring to FIG. 4, when the system input value PIN becomes a linearization input threshold value PIN—th, an input greater than or equal to a saturation input power level PIN—PA—sat is applied to the power amplifier by an output of the DPD. Accordingly, an output of the power amplifier after the linearization is saturated.
When an inverse function is determined via input/output characteristics of the saturated power amplifier, DPD output values steeply rise in a region exceeding PIN—th, as illustrated in the dotted line C of the drawing.
Accordingly, an input value of the power amplifier increases again, and this process iterates, such that a saturation region of the amplifier continues to expand.
When an input value of the amplifier continues to rise, an average input power of the amplifier increases, and consequently, an average output power of the amplifier increases and a divergence phenomenon is caused, creating instability in the digital pre-distortion transmission system.
As described above, in a conventional digital pre-distortion system, when an input greater than a predetermined level is given, an input peak level of a power amplifier gradually increases, when saturation occurs at the power amplifier. Consequently, an average output power rises, causing a divergence phenomenon.
The divergence phenomenon has a very bad influence on the stability and performance of a secure transmission system.